Many fuel systems, particularly fuel injection systems, rely on the fuel as a lubricant for one or more components thereof. Wear due to friction has been found to shorten the life of many of the components, such as fuel injection pumps and injectors, because of inadequate fuel lubricity.
Currently, a widely used procedure for evaluating fuel lubricity is to place a sample of a fuel to be tested in a test reservoir at the ambient atmospheric pressure and a specified test temperature. A fixed steel ball is held in a vertically mounted chuck and forced against a horizontally mounted stationary steel plate or disk under an applied load. The test ball is oscillated at a fixed frequency and stroke length parallel to the surface of the plate or disk, while the interface with the plate is fully immersed in the fuel. The metallurgies of the ball and plate, load, frequency, and stroke length are specified. Then, the ambient conditions during the test are used to correct the size of a wear scar or flat generated on the test ball to a standard set of ambient conditions, and the corrected wear scar diameter is used as a measure of the lubricating properties of the fuel. The test parameters, such as stroke length, oscillation frequency, temperature, applied load, and duration are specified under international testing standards. See for instance, the ISO/DIS 12156-1.3 test standard for the measurement of the lubricity of diesel fuel.
However, in many internal combustion engines, the fuel is maintained at elevated pressures, that is, pressures much greater than ambient atmospheric pressure conditions, for instance, 150 psi and greater, such that fuel lubricity is an important operating factor when selecting a fuel. Further, it has been found that fuel lubricity values determined under ambient atmospheric pressure conditions do not necessarily serve as an accurate predictor of actual fuel lubricity at the elevated actual operating pressure conditions.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.